Mineral Elements in Root of Wild Saposhnikovia divaricata and Its Rhizosphere Soil

Mineral elements are important components of medicinal herbs, and their concentrations are affected by many factors. In this study, Ca, Mg, Na, K, Fe, Mn, Cu, and Zn concentrations in wild Saposhnikovia divaricata and its rhizosphere soil collected from seven locations at two different times in China were measured, and influences of rhizosphere soil on those minerals in plant were evaluated. The results showed that mean concentrations of eight minerals in plant samples decreased in the order: Ca > Mg > Na > K > Fe > Zn > Mn > Cu, and those in the soil samples followed the following order: Na > Fe > Ca > K > Mg > Mn > Zn > Cu. Mean concentrations of Ca, Na, Mg, and K in plants were higher than those in soils, while higher mean concentrations of the other four minerals were found in soils. It was found that there was a positive correlation of Mg, Na, and Cu concentrations in the plant with those in the soil respectively, but a negative correlation of Mn concentration in plant with that in the soil. Except Ca, K, and Mn, the other five minerals in plant were all directly affected by one or more chemical compositions of soil. The results also indicate that pH value and concentrations of total nitrogen, Mg, Mn, and Cu in soil had significant correlations with multimineral elements in plant. In a word, mineral elements uptake of S. divaricata can be changed by adjusting the soil fertility levels to meet the need of appropriate quality control of S. divaricata.     

Rare earth elements in forest-floor herbs as related to soil conditions and mineral nutrition

Mixtures of rare earth elements (REEs) in fertilizers are widely used in Chinese agriculture to improve crop nutrition. REE concentrations in wild-growing plants, especially herbs, are little known. This study describes differences in the concentrations and proportions of REEs in eight forest-floor herbaceous plants and relates these differences to soil and mineral nutrient conditions. REEs studied were yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Leaf concentrations of sum REEs differed more than one order of magnitude between species, being highest in Anemone nemorosa (10.1 nmol/g dry mass) and lowest in Convallaria majalis (0.66 nmol/g) from the same site. Leaf concentrations of all REEs correlated positively (p<0.001), as did sum REE with calcium (Ca) and strontium (Sr) concentrations (p<0.001). A negative relationship (r=−0.83, (p<0.001) was measured between phosphorus (P) concentrations and sum REE concentrations in leaves. However, the proportions of the single REEs in the REE sum differed among species. In A. nemorosa, 57% of the molar REE sum was taken by Y+La, and only 21% by Ce. The other extreme was Maianthemum bifolium, with 37% La+Y and 41% Ce. These two species had 2.7–3.0% of the REE sum as heavier lanthanides, compared to 4.1–5.2% in the six other species. No clear relationship between soil properties or REE contents and leaf REE concentrations was detected. For La, however, an overrepresentation in leaves prevailed throughout all species compared to soils, whereas particularly Nd, Sm, and Tb had a lower proportion in the leaves of all species than in their soils. Possible uptake mechanisms of REEs in plants are discussed.     

Boron induced changes in biochemical constituents,enzymatic activities,and growth performance of wheat

The present study investigates how excess boron (B) affects and alters the biochemical constituents and enzymatic activities of wheat (Triticum aestivum var. ‘Raj 4037’), consequently leading to reduced plant growth and yield. Plants were raised in soils supplemented with various concentrations of B (0, 1, 2, 4, 8, 16, and 32 µg B g^?1 soil). Biochemical constituents including soluble leaf protein contents, total phenol contents, soluble sugar contents, proline contents, enzymatic activities of peroxidase (POX), and nitrate reductase (NR) were analyzed. In addition, growth parameters namely shoot–root length, shoot–root fresh and dry weight, seed number and seed weight were analyzed to assess the impact of B toxicity. Results indicate that change in biochemical constituents were correlated with B treatments. Boron concentrations beyond 4 µg g^?1 significantly increased soluble leaf protein contents, total phenol contents, soluble sugar contents, and proline contents. The POX activity was found to be positively correlated with B treatments. B significantly affects nitrogen metabolism and nitrate accumulation which is reflected by the downregulation of NR activity at higher B concentrations. B induced changes in physiological parameters of the plant which subsequently led to the reduction in growth, biomass production, and yield attributes. Out of the various concentrations of B, 8 µg g^?1 was moderately toxic while 16 and 32 µg g^?1 generated high toxicity and induced B stress response to confer tolerance in wheat. Further, a possible mechanism of B toxicity response in wheat is suggested.     

Rare earth elements and relation between their potential bioavailability and soil properties, Nidda catchment (Central Germany)

Rare earth elements (REEs) are widely used in industry and the entry of REEs into the pedosphere is assumed. Data about REEs in soils are scarce since only a few studies discuss their ecologically relevant behavior. Hence, we investigated total contents (aqua regia digestion) and potentially bioavailable contents (EDTA extraction) of REEs in soils from the Nidda catchment in Hesse (Central Germany). The study site covers a 1,600 km² sized area and 232 soil samples from 63 soil profiles were examined. The total REE content varied considerably, ranging from 544 mg kg^?1 to 41 mg kg^?1 (mean 201.1 mg kg^?1) with a high proportion of light REEs. Highest REE contents were found in the soilscape VB, followed by LVB, WNE, T, WSW and BF with the smallest concentrations. With respect to the parent material the contents decreased in the following order: basalt > clay slate > loess > sandstone. On average 15.9% of the total REEs belong to the potentially bioavailable fraction. They range greatly by a factor of 100, between 1.3 and 171.3 mg kg^?1 (average 33.5 mg kg^?1). Remarkably, Yttrium has a maximum available proportion of 75%. In contrast, Ce showed the highest total contents with the smallest potentially bioavailable proportion of all elements. Regression analyses established relation between soil properties and the potential bioavailability of REEs. Around 53% (range from 29.9 to 76.8%) of the REE’s potential bioavailability variations could be explained by the chosen variables (pH, clay and C_org contents and the total element concentrations). Occurrence patterns and concentrations of REEs lie within the range of the results found in the available literature. Bioavailability is linked to soil properties and varies greatly according to the individual element. In comparison with the chosen soil properties the pH value shows the least impact on bioavailability.     

Variation in seedling growth of 11 perennial legumes in response to phosphorus supply

Phosphorus (P) deficiency is a major problem for Australian agriculture. Development of new perennial pasture legumes that acquire or use P more efficiently than the current major perennial pasture legume, lucerne (Medicago sativa L.), is urgent. A glasshouse experiment compared the response of ten perennial herbaceous legume species to a series of P supplies ranging from 0 to 384 µg g^?1 soil, with lucerne as the control. Under low-P conditions, several legumes produced more biomass than lucerne. Four species (Lotononis bainesii Baker, Kennedia prorepens F.Muell, K. prostrata R.Br, Bituminaria bituminosa (L.) C.H.Stirt) achieved maximum growth at 12 µg P g^?1 soil, while other species required 24 µg P g^?1. In most tested legumes, biomass production was reduced when P supply was ≥192 µg g^?1, due to P toxicity, while L. bainesii and K. prorepens showed reduced biomass when P was ≥24 µg g^?1 and K. prostrata at ≥48 µg P g^?1 soil. B. bituminosa and Glycine canescens F.J.Herm required less soil P to achieve 0.5 g dry mass than the other species did. Lucerne performed poorly with low P supply and our results suggest that some novel perennial legumes may perform better on low-P soils.     

Contents and the biogeochemical characteristics of rare earth elements in wheat seeds

Contents of fifteen rare earth elements (REEs) in the seeds ofsixty breeds of wheat collected from seed bank were measured byinductively coupled plasma mass spectrometry (ICP-MS). The distributionpatterns of contents of REEs in wheat seeds (n = 58) wereobserved and compared with their average level in soils (n =364). Differences among regions and between spring and winter wheat weretested. Comparison with literature data was also made. The results showthat the content of REEs in wheat seed ranges between10^–11 g · g^–1 and10^–8 g · g^–1, 3–4 orderof magnitudes lower than that in soils. The distribution patterns arethat light REEs enriched and the Eu-anomaly is weak, similar to the soilcase. No obvious differences were found among different regions andbreeds. The data obtained in this study represent the contents of thefifteen REEs in wheat seeds.     

Mycorrhizal symbiosis and phosphorus fertilization effects on Zea mays growth and heavy metals uptake

Arbuscular mycorrhizal fungi (AMF) can promote plant growth and reduce plant uptake of heavy metals. Phosphorus (P) fertilization can affect this relationship. We investigated maize (Zea mays L.) uptake of heavy metals after soil AMF inoculation and P fertilization. Maize biomass, glomaline and chlorophyll contents and uptake of Fe, Mn, Zn, Cu, Cd and Pb have been determined in a soil inoculated with AMF (Glomus aggregatum, or Glomus intraradices) and treated with 30 or 60 µg P-K₂HPO₄ g^?1 soil. Consistent variations were found between the two mycorrhizal species with respect to the colonization and glomalin content. Shoot dry weight and chlorophyll content were higher with G. intraradices than with G. aggregatum inoculation. The biomass was highest with 30 µg P g^?1 soil. Shoot concentrations of Cd, Pb and Zn decreased with G. aggregatum inoculation, but that of Cd and Pb increased with G. intraradices inoculation. Addition of P fertilizers decreased Cd and Zn concentrations in the shoot. AMF with P fertilization greatly reduced maize content of heavy metals. The results provide that native AMF with a moderate application rate of P fertilizers can be exploited in polluted soils to minimize the heavy metals uptake and to increase maize growth.     

The invasive Sorghum halepense harbors endophytic N2-fixing bacteria and alters soil biogeochemistry

Exotic plants invading new habitats frequently initiate broad changes in ecosystem functioning. Sorghum halepense is an invasive grass capable of growing in nitrogen (N)-poor tallgrass prairie soils that creates near monocultures in once phylogenetically diverse-communities. The biogeochemistry of soils invaded by S. halepense was compared to that of un-invaded native prairie soils. Invaded soils contained two to four times greater concentrations of alkaline metals, micronutrients, and essential plant nutrients than native prairie soils. The notable exception was Ca⁺², which was always significantly lower in invaded soils. The N-content of S. halepense above-ground biomass was 6.4 mg g^?1 (320 mg N plant^?1) and suggested a supplemental N source supporting plant growth. Altered soil biogeochemistry in invaded areas coupled with high above-ground biomass in N-poor soils suggested N₂-fixing activity associated with S. halepense. Nitrogenase activity of plant tissues indicated that N₂-fixation was occurring in, and largely restricted to, S. halepense rhizomes and roots. A culture approach was used to isolate these N₂-fixing bacteria from plant tissues, and 16S rRNA gene sequencing was used to identify these bacterial isolates. Nitrogenase activity of bacterial isolates indicated several were capable of N₂-fixation. In addition to N₂-fixation, other roles involved in promoting plant growth, namely mobilizing phosphorus and iron chelation, are known for closest matching relatives of the bacterial isolates identified in this work. Our results indicate that these plant growth-promoting bacteria may enhance the ability of S. halepense to invade and persist by altering fundamental ecosystem properties via significant changes in soil biogeochemistry.     

Accumulation of arsenic in soil and rice under wetland condition in Bangladesh

Shallow tube well (STW) water, often contaminated with arsenic (As), is used extensively in Bangladesh for irrigating rice fields in the dry season, leading to potential As accumulation in soils. In the current study the consequences of arsenic from irrigation water and direct surface (0–15 cm) soil application were studied under field conditions with wetland rice culture over 2 years. Twenty PVC cylinders (30-cm length and 30-cm diameter) were installed in field plots to evaluate the mobility and vertical distribution of soil As, As mass balance, and the resulting influences on rice yield and plant-As concentration in Boro (dry season) and transplanted (T.) Aman (wet season) rice over the 2-year growth cycle. Treatments included irrigation-water As concentrations of 0, 1 and 2 mg L^?1 (Boro season only) and soil-As concentrations of 10 and 20 mg kg^?1. Following the 2-year cropping sequence the major portion (39.3–47.6%) of the applied arsenic was retained within the rooting zone at 0–15 cm depth, with 14.7–19.5% of the total applied As at the 5–10 cm and 10–15 cm soil depths compared to 1.3–3.6% at the 35–40 cm soil depth. These results indicate the relatively low mobility of applied As and the likely continued detrimental accumulation of As within the rooting zone. Arsenic addition in either irrigation water or as soil-applied As resulted in yield reductions from 21 to 74 % in Boro rice and 8 to 80 % in T. Aman rice, the latter indicating the strong residual effect of As on subsequent crops. The As concentrations in rice grain (0.22 to 0.81 µg g^?1), straw (2.64 to 12.52 µg g^?1) and husk (1.20 to 2.48 µg g^?1) increased with increasing addition of As. These results indicate the detrimental impacts of continued long-term irrigation with As-contaminated water on agricultural sustainability, food security and food quality in Bangladesh. A critical need exists for the development of crop and water management strategies to minimize potential As hazard in wetland rice production.     

Leaf silicon content in banana (Musa spp.) reveals the weathering stage of volcanic ash soils in Guadeloupe

Several plant species accumulate silicon, which is taken up by roots in soil solution. The Si concentration in soil solution can be governed by silicate dissolution and formation, and thus soil constitution. Here, we study the Si leaf content of mature banana plants (Musa acuminata cv Grande Naine) cropped on soils derived from andesitic ash in Guadeloupe through standard foliar analysis. The soils strongly differ in weathering stage and total Si content. The most desilicated soils (Andosol–Nitisol–Ferralsol) occur in the wettest areas, on the Eastern slopes (Es) of the volcano exposed to rain bearing winds. Least weathered soils (Andosol–Cambisol) occur on Western slopes (Ws). The average leaf Si concentration ranges from 2.7 to 3.9 g kg^?1 for bananas cropped in Es soils, and from 7.7 to 9.6 g kg^?1 in Ws soils. The leaf Si concentrations are lowest for the Es gibbsite-rich Andosols and Ferralsols. The leaf Si concentration is positively correlated with soil CaCl₂-extractable Si content, soil Si content and total reserve in weatherable minerals. The silicon content of banana leaves thus reveals the weathering stage of volcanic ash soils in Guadeloupe.     

Accumulation and fractionation of rare earth elements (REEs) in the naturally grown Phytolacca americana L. in southern China

The widespread use of rare earth elements (REEs) has resulted in problems for soil and human health. Phytolacca americana L. is a herbaceous plant widely distributed in Dingnan county of Jiangxi province, China, which is a REE mining region (ion absorption rare earth mine) and the soil has high levels of REEs. An investigation of REE content of P. americana growing naturally in Dingnan county was conducted. REE concentrations in the roots, stems, and leaves of P. americana and in their rhizospheric soils were determined. Results showed that plant REEs concentrations varied among the sampling sites and can reach 1040 mg/kg in the leaves. Plant REEs concentrations decreased in the order of leaf > root > stem and all tissues were characterized by a light REE enrichment and a heavy REE depletion. However, P. americana exhibited preferential accumulation of light REEs during the absorption process (from soil to root) and preferential accumulation of heavy REEs during the translocation process (from stem to leaf). The ability of P. americana to accumulate high REEs in the shoot makes it a potential candidate for understanding the absorption mechanisms of REEs and for the phytoremediation of REEs contaminated soil.     

Regulation of heavy metal concentrations in cereal grains from uranium mine soils

Aims

Along a gradient of diminishing heavy metal (HM) concentrations formed by local inclusions of uranium mine soils into non-contaminated cropland, duplicate 1-m² plots of 3 winter wheat cvs. (Akteur E, Brilliant A, and Bussard E) were established at 3 positions within a winter rye (cv. Visello) culture. It was the goal to determine permissible soil HM concentrations tolerated by cereal cvs. with variable excluder properties, and regulatory mechanisms which optimize the concentrations of essential minerals and radionuclide analogues in viable seeds from geologically related soils with diverging HM content.

Methods

Total metal concentrations / nitrogen species in soils, shoots, and mature grains were determined by ICP-MS / spectrophotometry, and Kjeldahl analyses.

Results

No non-permissible concentrations in grains of the 4 cereal cvs. were caused by elevated but aged total soil resources (mg kg^-1 DW) in As (156); Cu (283); Mn (2,130); Pb (150); and in Zn (3,005) in the case of Bussard although CdCuZn elicited phytotoxicity symptoms. Uranium (41) contaminated grains of Akteur and Brilliant but not of Bussard and Visello due to their excluder properties. The concentration in Cd (41) had to be reduced to 20/2 mg kg^-1 for the production by excluder cvs. of fodder/food grains. Cultivars excluding both HM and radionuclide analogues such as BaCsSr synchronously were not identified. Whereas plant tissue concentrations in the metalloprotein-associated elements CdCoCuMnNiZn rise and fall generally with N_org, grains of the wheat cvs. differed too little in N_org to designate variations in their metal acquisition rates solely as protein-regulated. Wheat grains confined nevertheless the concentrations in Cu to 11–14 mg kg^-1 although the respective soil concentrations varied by factor 19. Grain deposition in CaFeMn(Zn) and in nuclides followed the same rules.

Conclusions

It is hypothesized that cereals down-/up-regulate grain:soil transfer rates from soils with excessive/deficient trace metal resources to equip viable seeds with an optimum but not maximum in essential minerals. Positive correlations between metal concentrations in planta to those in soil can thereby be lost.     

Ferrous Iron Treatment of Soils Contaminated with Arsenic-Containing Wood-Preserving Solution

This article discusses the results of efforts to reclaim As-contaminated soil from a former timber-treating plant. The study site, commonly referred to as the Rocker Timber Framing site, is located along Silver Bow Creek approximately 7 miles west of the Butte Mining District, MT, USA. The plant operations resulted in contamination of the soils with a highly caustic solution containing 5% As (III). Contaminated soil resulted in the groundwater plumes that contained up to 25?mg L^?1 As, with As (V) being the predominant species. The objective of this study was to evaluate the effectiveness of Fe (II) treatment for remediation of As-contaminated soils. Laboratory-treatability studies were conducted on samples of saturated zone (AS1) and va-dose zone (AV1) soils. The AS1 soil was a mixture of coarse alluvium and potentially some mill tailings from adjacent mining operations. The AV1 soil consisted primarily of fill, including soil, construction debris, and timber fragments. Initial concentrations of total As in AS1 and AV1 soils were 683 and 4814?µg kg^?1, respectively. Water-soluble As concentrations were 15.4 and 554?µg L^?1, respectively, in a 20:1 solution to soil extract. Batch equilibration were performed by placing 10?g of soil into 20 vessels and adding increasing amounts of FeSO₄.7H₂O. Amendment increments were made as multiples of molar ratios of total As present in each soil. Treatability studies were run with and without a pH buffer of CaCO₃ (added at a 2:1 molar ratio to the FeSO₄.7H₂O treatment). Solution concentrations of As in the AS1 and AV1 soils (without CaCO₃) decreased from 554 to 15.4?µ L^?1 and 3802 to 0.64?µ L^?1, respectively, as the Fe:As molar ratios increased from 0 to 2, whereas for the AS1 soil the solution As concentration increased at the Fe:As molar ratios >2 and reverse trend was observed for the AV1 soils. The decrease in As solution concentration for the AS1 soil is attributable to the dramatic decrease in soil pH with increasing Fe:As molar ratios. In the case of soils treated with CaCO₃, the solution concentrations decreased from 564 to 0.65?µg L^?1 and 3790 to 0.79?µg L^?1 for the AS1 and AV1 soils, respectively,as the Fe:As molar ratios increased from 0 to 50. Generally, in both the soils, the CaCO₃-treated soil contained significantly more solution As compared with the non-CaCO₃-treated soil at the comparable Fe:As molar ratios. This is attributable to higher solution pH on CaCO₃ treatment. Our rapid engineering study indicates that treating both the soils with Fe:As molar ratio of 2 lowered the As water quality limit to <50?µL^?1, whereas treating the AS1 and AV1 soils with Fe:As molar ratio of 2 and 3, respectively, lowered the As water quality limit to ≤15?µg L^?1. The concentrations of the Cu and Zn were below the instrument detection limits for the AS1 and AV1 soils without CaCO₃ treatment. Sequential extraction of Fe-treated soils illustrated that As was relatively stable. Less than 1% of the As was extractable using a modified TCLP approach and <70% of the As was extractable using a harsh acid modified hydroxylamine hydrochloride extraction.     

The reserve of weatherable primary silicates impacts the accumulation of biogenic silicon in volcanic ash soils

Banana plantlets (Musa acuminata cv Grande Naine) cultivated in hydroponics take up silicon proportionally to the concentration of Si in the nutrient solution (0–1.66 mM Si). Here we study the Si status of banana plantlets grown under controlled greenhouse conditions on five soils developed from andesitic volcanic ash, but differing in weathering stage. The mineralogical composition of soils was inferred from X-ray diffraction, elemental analysis and selective chemical/mineralogical extractions. With increasing weathering, the content of weatherable primary minerals decreased. Conversely, clay content increased and stable secondary minerals were increasingly dominant: gibbsite, Fe oxides, allophane, halloysite and kaolinite. The contents of biogenic Si in plant and soil were governed by the reserve of weatherable primary minerals. The largest concentrations of biogenic Si in plant (6.9–7 g kg⁻¹) and soil (50–58 g kg⁻¹) occurred in the least weathered soils, where total Si content was above 225 g kg⁻¹. The lowest contents of biogenic Si in plant (2.8–4.3 g kg⁻¹) and soil (8–31 g kg⁻¹) occurred in the most weathered desilicated soils enriched with secondary oxides and clay minerals. Our data imply that soil weathering stage directly impacted the soil-to-plant transfer of silicon, and thereby the stock of biogenic Si in a soil–plant system involving a Si-accumulating plant. They further imply that soil type can influence the silicon soil–plant cycle and its hydrological output.     

Nutrient status of black spruce (Picea mariana [Mill.] BSP) forest soils dominated by Kalmia angustifolia L.

A study was conducted to determine soil chemistry in an uncut black spruce (Picea mariana) forest with and without the ericaceous understory shrub Kalmia angustifolia, as well as on a cut black spruce forest currently dominated by Kalmia. The organic (humus) and mineral (Ae, upper and lower B horizons) soils associated with Kalmia from uncut and cut forests, and non-Kalmia soils from uncut forest, were analyzed for selected soil properties. In general, mineral soils (B horizon) associated with Kalmia in uncut forest have lower values for organic matter (3.25%), organic nitrogen (0.66 mg·g⁻¹), Fe³⁺ (95.4 μg·g⁻¹) and Mn²⁺ (9 μg·g⁻¹), and higher values for pH (4.12) and Ca²⁺ (27 μg·g⁻¹) compared to non-Kalmia (organic matter, 3.43%; organic-N, 1.15 mg·g⁻¹; Fe³⁺, 431 μg·g⁻¹; Mn²⁺, 23.2 μg·g⁻¹; pH, 3.14; Ca²⁺, 15.6 μg·g⁻¹) and cut black spruce-Kalmia (organic matter, 3.74%; organic-N, 0.94 mg·g⁻¹; Fe³⁺, 379 μg·g⁻¹; Mn²⁺, 27 μg·g⁻¹; pH, 2.87; Ca²⁺, 25.2 μg·g⁻¹) forest. The high C:N ratio in Kalmia mineral soil from upper B (29.73) and lower B (identified as B+) (33.08) in uncut black spruce forest was recorded compared to non-Kalmia soils in B (18.17) and B+ (17.05) horizons in uncut black spruce forest. Phenolics leached out from Kalmia litter were lower in Kalmia associated soils than the non-Kalmia soils from the uncut forest, and Kalmia associated soils from the cut forest area. Results indicate that soils associated with Kalmia were nutrient poor particularly for nitrogen, phosphorus, iron and manganese, and provide some basis for the hypothesis that Kalmia has dominated microsites that were nutrient poor prior to Kalmia colonization.     

Toxicity of Cd to signal grass (Brachiaria decumbens Stapf.) and Rhodes grass (Chloris gayana Kunth.)

Given that Cd accumulates within plant tissues to levels that are toxic to animals, it is necessary to understand the role of plants in highly Cd-contaminated systems and their subsequent impact on the health of animals. A solution culture experiment was conducted to elucidate the effects of increasing Cd²⁺ activity ({Cd²⁺}) on growth of Rhodes grass (Chloris gayana Kunth.) and signal grass (Brachiaria decumbens Stapf.). The shoot and root fresh mass of both Rhodes grass and signal grass was reduced by 50% at ca. 0.5 µM {Cd²⁺}. Elevated {Cd²⁺} resulted in a significant decrease in the tissue Mn concentration for both the shoots and roots, and caused a chlorosis of the veins in the shoots. Root hair growth was prolific even at high {Cd²⁺}, thus root hair growth appeared to be less sensitive to elevated Cd than was root growth per se. The critical shoot tissue concentrations (50% reduction in growth), 230 µg g^?1 for Rhodes grass and 80 µg g^?1 for signal grass, exceeded the maximum level of Cd tolerated in the diet of animals (ca. 5 µg g^?1). When assessing the risk associated with the revegetation of Cd-contaminated sites with Rhodes grass or signal grass, careful consideration must be given, therefore, to the transfer of toxic concentrations of Cd to grazing animals and through the wider food chain.     

Impact of commonly used agrochemicals on bacterial diversity in cultivated soils

The effects of three selected agrochemicals on bacterial diversity in cultivated soil have been studied. The selected agrochemicals are Cerox (an insecticide), Ceresate and Paraquat (both herbicides). The effect on bacterial population was studied by looking at the total heterotrophic bacteria presence and the effect of the agrochemicals on some selected soil microbes. The soil type used was loamy with pH of 6.0–7.0. The soil was placed in opaque pots and bambara bean (Vigna subterranean) seeds cultivated in them. The agrochemicals were applied two weeks after germination of seeds at concentrations based on manufacturer’s recommendation. Plant growth was assessed by weekly measurement of plant height, foliage appearance and number of nodules formed after one month. The results indicated that the diversity index (Di) among the bacteria populations in untreated soil and that of Cerox-treated soils were high with mean diversity index above 0.95. Mean Di for Ceresate-treated soil was 0.88, and that for Paraquattreated soil was 0.85 indicating low bacterial populations in these treatment-type soils. The study also showed that application of the agrochemicals caused reduction in the number of total heterotrophic bacteria population sizes in the soil. Ceresate caused 82.50% reduction in bacteria number from a mean of 40 × 10⁵ cfu g⁻¹ of soil sample to 70 × 10⁴ cfu g⁻¹. Paraquat-treated soil showed 92.86% reduction, from a mean of 56 × 10⁵ cfu g⁻¹ to 40 × 10⁴ cfu g⁻¹. Application of Cerox to the soil did not have any remarkable reduction in bacterial population number. Total viable cell count studies using Congo red yeast-extract mannitol agar indicated reduction in the number of Rhizobium spp. after application of the agrochemicals. Mean number of Rhizobium population numbers per gram of soil was 180 × 10⁴ for the untreated soil. Cerox-treated soil recorded mean number of 138 × 10⁴ rhizobial cfu g⁻¹ of soil, a 23.33% reduction. Ceresate- and Paraquat-treated soils recorded 20 × 10⁴ and 12 × 10⁴ cfu g⁻¹ of soil, respectively, representing 88.89% and 93.33% reduction in Rhizobium population numbers. Correspondingly, the mean number of nodules per plant was 44 for the growth in untreated soil, 30 for the plant in the Cerox-treated soil, 8 for the plant in Paraquat-treated soil and 3 for the plant in Ceresate-treated soil. The study has confirmed detrimental effect of insecticide on bacterial populations in the soil. Total heterotrophic counts, rhizobial counts as well as the number of nodules of all samples taken from the chemically treated soils were all low as compared to values obtained for the untreated soil. However, the effect of the insecticide was minimal in all cases as compared to the effects of the herbicides on the soil fauna. Indiscriminate use of agrochemicals on farms can therefore affect soil flora and subsequently food production.     

Soil minerals affect taxon-specific bacterial growth

Secondary minerals (clays and metal oxides) are important components of the soil matrix. Clay minerals affect soil carbon persistence and cycling, and they also select for distinct microbial communities. Here we show that soil mineral assemblages—particularly short-range order minerals—affect both bacterial community composition and taxon-specific growth. Three soils with different parent material and presence of short-range order minerals were collected from ecosystems with similar vegetation and climate. These three soils were provided with ¹⁸O-labeled water and incubated with or without artificial root exudates or pine needle litter. Quantitative stable isotope probing was used to determine taxon-specific growth. We found that the growth of bacteria varied among soils of different mineral assemblages but found the trend of growth suppression in the presence of short-range order minerals. Relative growth of bacteria declined with increasing concentration of short-range order minerals between 25–36% of taxa present in all soils. Carbon addition in the form of plant litter or root exudates weakly affected relative growth of taxa (p = 0.09) compared to the soil type (p < 0.01). However, both exudate and litter carbon stimulated growth for at least 34% of families in the soils with the most and least short-range order minerals. In the intermediate short-range order soil, fresh carbon reduced growth for more bacterial families than were stimulated. These results highlight how bacterial-mineral-substrate interactions are critical to soil organic carbon processing, and how growth variation in bacterial taxa in these interactions may contribute to soil carbon persistence and loss.Subject terms: Biogeochemistry, Microbial ecology, Biogeochemistry     

The vegetation of ultrabasic soils on the Isle of Rhum II. The causes of the debris

Summary

The relationship between the Rhum ultrabasic skeletal soils and their debris vegetation was investigated by plant analyses and field and laboratory experiments. Samples of Agrostis vinealis, Arenaria norvegica ssp norvegica, Calluna vulgaris, Festuca vivipara, Plantago maritima and Racomitrium lanuginosum from these soils usually had low concentrations of potassium and calcium, and high concentrations of sodium, magnesium (and high Mg/Ca quotients), iron and nickel. There were instances of very high iron concentrations (up to 22.4mg g^?1 in Plantago maritima), very high Mg/Ca quotients (up to 27.8 in Arenaria novegica spp. norvegica) and high nickel concentrations (up to 0.48mg g^?1 in Plantago maritima). A nutrient addition experiment which was set up in 1965 on an exposed barren area had in 1982 over twice as many species as originally recorded and a nearly complete plant cover. Shorter-term work has confirmed that nutrient availability limits the ultrabasic vegetation. An experiment on Agrostis vinealis in simulated soil solutions showed that a gabbro (non-ultrabasic) clone had a higher R.G.R. (relative growth rate), even at the higher of two experimental Mg/Ca quotients, than two peridotite (ultrabasic) clones and was moreover not significantly affected by the higher nickel concentration used. It is concluded that the low vegetation cover on the skeletal soils is maintained by low soil nutrients which might interact with the coarsely sandy texture in exacerbating the effects of periodic drought and frost heaving. There is no unequivocal evidence for plant toxicities associated with high magnesium or nickel in the Rhum soils.     

Contamination of Phthalate Esters (PAEs) in Typical Wastewater-Irrigated Agricultural Soils in Hebei,North China

The Wangyang River (WYR) basin is a typical wastewater irrigation area in Hebei Province, North China. This study investigated the concentration and distribution of six priority phthalate esters (PAEs) in the agricultural soils in this area. Thirty-nine soil samples (0–20 cm) were collected along the WYR to assess the PAE residues in soils. Results showed that PAEs are ubiquitous environmental contaminants in the topsoil obtained from the irrigation area. The concentrations of Σ₆PAEs range from 0.191 μg g⁻¹ dw to 0.457 μg g⁻¹ dw with an average value of 0.294 μg g⁻¹ dw. Di(2-ethylhexyl) phthalate (DEHP) and di-n-butyl phthalate (DnBP) are the dominant PAE species in the agricultural soils. Among the DEHP concentrations, the highest DEHP concentration was found at the sites close to the villages; this result suggested that dense anthropogenic activities and random garbage disposal in the rural area are possible sources of PAEs. The PAE concentrations were weakly and positively correlated with soil organic carbon and soil enzyme activities; thus, these factors can affect the distribution of PAEs. This study further showed that only dimethyl phthalate (DMP) concentrations exceeded the recommended allowable concentrations; no remediation measures are necessary to control the PAEs in the WYR area. However, the PAEs in the topsoil may pose a potential risk to the ecosystem and human health in this area. Therefore, the exacerbating PAE pollution should be addressed.